linux-kernel - Re: [PATCH] memcg: expose root cgroup's memory.stat

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Date:   Fri, 15 May 2020 17:13:12 -0700
From:   Shakeel Butt <shakeelb@...gle.com>
To:     Johannes Weiner <hannes@...xchg.org>
Cc:     Roman Gushchin <guro@...com>, Michal Hocko <mhocko@...nel.org>,
        Mel Gorman <mgorman@...e.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Yafang Shao <laoar.shao@...il.com>,
        Linux MM <linux-mm@...ck.org>,
        Cgroups <cgroups@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] memcg: expose root cgroup's memory.stat

On Fri, May 15, 2020 at 11:09 AM Johannes Weiner <hannes@...xchg.org> wrote:
>
> On Fri, May 15, 2020 at 10:49:22AM -0700, Shakeel Butt wrote:
> > On Fri, May 15, 2020 at 8:00 AM Roman Gushchin <guro@...com> wrote:
> > > On Fri, May 15, 2020 at 06:44:44AM -0700, Shakeel Butt wrote:
> > > > On Fri, May 15, 2020 at 6:24 AM Johannes Weiner <hannes@...xchg.org> wrote:
> > > > > You're right. It should only bypass the page_counter, but still set
> > > > > page->mem_cgroup = root_mem_cgroup, just like user pages.
> > >
> > > What about kernel threads? We consider them belonging to the root memory
> > > cgroup. Should their memory consumption being considered in root-level stats?
> > >
> > > I'm not sure we really want it, but I guess we need to document how
> > > kernel threads are handled.
> >
> > What will be the cons of updating root-level stats for kthreads?
>
> Should kernel threads be doing GFP_ACCOUNT allocations without
> memalloc_use_memcg()? GFP_ACCOUNT implies that the memory consumption
> can be significant and should be attributed to userspace activity.
>
> If the kernel thread has no userspace entity to blame, it seems to
> imply the same thing as a !GFP_ACCOUNT allocation: shared public
> infrastructure, not interesting to account to any specific cgroup.
>
> I'm not sure if we have such allocations right now. But IMO we should
> not account anything from kthreads, or interrupts for that matter,
> /unless/ there is a specific active_memcg that was set by the kthread
> or the interrupt.

I totally agree with you but I think your response is about memory
charging in IRQ/kthread context, a topic of a parallel patch from
Zefan Li at [1].

Here we are discussing stats update for kthreads e.g. should we update
root memcg's MEMCG_KERNEL_STACK_KB stat when we allocate stack for
kernel threads?

[1] http://lkml.kernel.org/r/3a721f62-5a66-8bc5-247b-5c8b7c51c555@huawei.com